Field of Invention
The present invention relates to a fault diagnosis method of a SCR system and an apparatus thereof, and more particularly, to a fault diagnosis method of a SCR system and an apparatus thereof reflecting states of a lean NOx trap and an urea tank in a lean NOx trap and a SCR complex post-processing system.
Description of Related Art
There have been the problems of serious environmental pollutions by emissions exhausted from an internal combustion engine of vehicles using fossil fuel such as gasoline or diesel. In particular, the discharging of soot, nitrogen oxide (NOx) and fine dust including soot exhausted from diesel vehicles such as buses and trucks are perceived as a serious problem, and thus, each country controls strictly emission of exhaust gas by preparing related regulations in order to solve exhaust gas problems of such above diesel vehicles.
Generally, the exhaust system of a diesel engine is provided with an exhaust gas post-processing unit such as DOC (Diesel Particulate matter Filter), DPF (Diesel Particulate matter Filter), SCR (Selective Catalyst Reduction) and LNT (Lean NOx Trap) and so on in order to reduce the pollutants contained in the exhaust gas such as carbon monoxide (CO), hydrocarbon (HC), particulate matter, and nitrogen oxides (NOx), and so on.
Among them, an exhaust gas post-processing device (hereinafter, referred to as “SCR system”) using SCR may function as injecting a reducing agent such as urea inside an exhaust pipe, thereby reducing nitrogen oxides in the exhaust gas to nitrogen and oxygen. In other words, the above SCR system injects a reducing agent such as urea into an exhaust pipe, and then the reducing agent is converted to ammonia (NH3) by the heat of exhaust gas, thereby reducing nitrogen oxides (NOx) to nitrogen gas (N2) and water (H2O) by a catalytic reaction of nitrogen oxides (NOx) and ammonia in the exhaust gas via the SCR catalyst.
A complex post-processing system equipped with ‘a lean NOx trap (LNT) and SCR (Selective Catalyst Reduction) in series’, as a new technology for diesel vehicles to respond to the recently intensified regulation of NOx exhaust gas, has emerged (refer to FIG. 4). Under this lean NOx trap and SCR complex post-processing system, the amount of post-injected fuel increases during the regeneration (DeNOx) of the lean NOx trap, thereby the air-fuel ratio goes down. In the NOx sensor disposed at the front and rear end of SCR to measure the mass of NOx, the measurement error increases abruptly in the condition of low air-fuel ratio due to the nature of the sensor such that the measured value in this interval cannot be reliable. However, since there was no function in a related art to consider whether the lean NOx trap is regenerated at the time of entering into monitoring the SCR purification efficiency, an erroneous detection on the purification efficiency of the SCR system may occur in the fuel-rich state of the exhaust gas.
Furthermore, if a urea tank is frozen state or cavity occurrence state, the urea supply is not smooth, thereby the purification efficiency of the SCR system decreases. However, there is no function in a related art to consider whether an urea tank is frozen state or cavity occurrence state at the time of entering into monitoring the SCR purification efficiency, thereby it may detect erroneously even the decrease of the purification efficiency in the SCR system due to a heat deterioration in the SCR catalyst or a breakdown in the urea supply line.
The information disclosed in this Background section is only for enhancement of understanding of the general background of the invention and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art already known to a person skilled in the art.